A semiconductor layer comprising a conductive polymer is generally used as the cathode of a solid electrolytic capacitor. For example, a known method for forming a semiconductor layer comprises immersing an anode body having a dielectric layer on the surface thereof in an electrolyte solution in which a monomer for forming a conductive polymer and a dopant are dissolved in water, and electropolymerizing the monomer. This method results in the formation of a semiconductor layer comprising a conductive polymer on the dielectric layer. In this method, for example, a compound having a thiophene skeleton is used as the monomer. Such a monomer is only slightly soluble in water. The amount of monomer dissolved in the electrolyte solution is not sufficient for the formation of a semiconductor layer.
The monomer is easily dissolved in an organic solvent. It is proposed to use an organic solvent, rather than water, as a solvent of an electrolyte solution for an electropolymerization (Patent Document 1). However, many organic solvents are easily evaporated, and it requires time and labor to maintain the liquid surface position. Moreover, an electrolyte solution using such a non-aqueous organic solvent has high electrical resistance, which causes a problem of low polymerization efficiency.
It is proposed to use a mixed solvent of water and an organic solvent as a solvent of an electrolyte solution. The solubility of the monomer in this mixed solvent is higher than that of a solvent using only water; however, the monomer cannot be dissolved in an amount required to form a semiconductor layer.
In order to increase the amount of monomer supplied, Patent Document 2 discloses a technique for adding a monomer in an amount exceeding the saturated solubility to a mixed solvent of water and an organic solvent, suspending a part of the monomer in the solvent, and performing electropolymerization while stirring the suspension.